A friend like that just isn't good for you, he's going to make you more conscious of your stuttering and if I were you I'd give him an ultimatum: No more laughing at it or we're done. It's not an unreasonable request, I'm 27 and I haven't come across one person who has treated me like that. With women it's actually a little different, I've had the occasional chuckle but I chalk it down to them not understanding/wanting to lighten up the situation. You need to show comfort with women so a chuckle isn't the worst thing. But seriously you can't be around someone like that all the time.